A preliminary study in Israel, believed to be the world's first, found that a fourth COVID-19 vaccine is not enough to prevent omicron infections.
Gili Regev-Yochay, director of Sheba Medical Center's Infectious Diseases Unit, said the trial studied the effect of the Pfizer booster after two weeks and the Moderna booster after one week, according to Reuters.
Regev-Yochay said the increase in antibodies from a fourth shot was "probably not enough for the Omicron."
"We know by now that the level of antibodies needed to protect and not to got infected from Omicron is probably too high for the vaccine, even if it's a good vaccine," she added.
Sheba Medical Center ran the trial on second booster shots among 150 of its staff members, and its findings were preliminary and unpublished, The Times of Israel reported. About 500,000 Israelis have been inoculated with a fourth dose as of Sunday.
Israel has led the push for vaccinations throughout the pandemic and was the first country to begin to offer booster shots to its population.
Despite the push for vaccinations, Israel reported nearly 12,000 new COVID-19 cases earlier this month, a record-setting figure for daily case rates since the start of the pandemic.
"There is no control of the omicron wave," Sharon Alroy-Preis, the Israeli health ministry's top public health official, said to a local news outlet at the time.